Die cast method and die cast machine

ABSTRACT

The present invention provides a die-casting method comprising the steps of evacuating a cavity  30  defined by dies of a die-casting machine to provide therein a vacuum of 100 millibar or less during a first period, injecting a reactive gas from a sleeve of the die-casting machine into the cavity  30  during a second period which has a partial overlap period with the first period and follows the overlap period, so as to increase the inner pressure of the cavity to atmospheric pressure or more, pouring a molten aluminum alloy M into the sleeve  40  while keeping the injection of the reactive gas, and subsequently moving a plunger  42  in the sleeve forward to forcibly inject the molten aluminum alloy from the sleeve into the cavity  30  while re-evacuating the cavity  30.  The evacuation, reactive-gas injection and re-evacuation operations can be performed with respective overlap periods therebetween. During the operation of supplying the molten aluminum alloy, air and water are effectively discharged to outside, and the reactive gas in an unreacted state is discharged to outside without being incorporated into the molten aluminum alloy. Thus, the present invention provides a die-cast product having a significantly reduced volume of incorporated gases and allows the die-cast product to be applied to functional members.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a die-casting method and machinefor manufacturing a die-cast product usable as not only structuralmembers but also functional members by virtue of its suppressed castdefects such as internal porosities or blowholes.

[0002] In a conventional die-casting method, molten aluminum or moltenaluminum alloy (hereinafter correctively referred to as “molten metal”)poured into a sleeve of a die-casting machine is forcibly injected intoa cavity defined in dies by a plunger in the sleeve. While most ofgases, such as air or water vapor, residing in the cavity are purgedfrom the cavity in conjunction with the injection of the moltenaluminum, a part of the gases can be undesirably left in the cavityafter the completion of the injection of the molten aluminum alloy.Particularly, a die assembly designed for a specific product having avery thin thickness or a complicated configuration can involve narrowedportions constraining fluid flow, and thereby it is difficult to achievecomplete exclusion of gases residing in its die cavity.

[0003] During a cooldown/solidification stage of the injected moltenaluminum in the dies, the residual gases in the cavity are took in themolten aluminum and incorporated into the resulting die-cast product asa factor of casting defects such as internal porosities or blowholes. Asa result, the obtained die-casting products have suffered frominferiority in mechanical properties such as strength or elongation,resulting in unfavorable evaluation for being inadequate to use asfunctional components, such as a scroll, piston, cylinder block,connecting rod or suspension member. If such casting defects caused bythe residual gases are suppressed, the die-casting method will haveenlarged range of applicable fields with its intrinsic excellentproductivity.

[0004] There has been known a vacuum die-casting method as one oftechniques intended to eliminate the adverse effect of the residualgases. In the vacuum die-casting method, a die cavity is evacuated priorto a forcible injection of molten aluminum in order to remove air fromthe cavity. However, in this method, the internal pressure of the cavityhas a limited vacuum ranging from 200 to 500 millibar and it ispractically impossible to obtain further reduced vacuum, because someambient air enters in the cavity through a gap between mating faces ofdies and another ambient air is additionally introduced in a sleeve of adie-casting machine during the operation of pouring the molten aluminuminto the sleeve. Thus, even though a product obtained from the vacuumdie-casting method has a reduced volume of incorporated air as comparedwith products from another conventional die-casting method, it is stillinvolved with the casting defects such as internal porosities caused bythe incorporated gases, and is thereby quite inadequate to use asfunctional components.

[0005] An oxygen die-casting method has been developed to eliminate thedisadvantage of the vacuum die-casting method (see Japanese PatentPublication No. 50-21143). In the oxygen die-casting method, oxygen gasis filled in a die cavity with a pressure of atmospheric pressure ormore to replace gases in the cavity by the oxygen gas. Thus, anexcessive part of the supplied oxygen gas is blown out of the cavitythrough a gap between mating faces of dies and a pour opening forpouring molten aluminum therethrough to prevent ambient air fromentering in the cavity through the gap and the pour opening. Thesupplied oxygen gas remaining in the cavity reacts with the moltenaluminum to form a fine structure of Al₂O₃ dispersed over the resultingdie-cast product without any adverse effect on the properties of theproduct.

[0006] However, even by supplying the oxygen gas into the cavity withthe pressure of atmospheric pressure or more, it is difficult tocompletely remove air from the cavity. Generally, a die cavity having acomplicated configuration leads to an increased volume of residual airtherein. More specifically, the die cavity having a complicatedconfiguration involves a narrowed portion incapable of receiving thereinthe supplied oxygen gas, and the narrowed portion keeps gases such asair and water vapor residing therein without replacing the gases by thesupplied oxygen gas. These residual gases are incorporated into theresulting die-cast product as a factor causing the casting defects.

[0007] The residual air in the cavity as a factor causing the castingdefects can be effectively replaced by oxygen gas by injecting theoxygen-gas simultaneously with the evacuation of the cavity (JapanesePatent Publication No. 57-140). However, even if the oxygen gas isinjected in synchronous with the evacuation of the cavity, water is noteffectively removed from the cavity. In fact, a die-cast productobtained from this technique is still involved with the cast defectscaused by the residual gases. A technique of injecting oxygen gas afterthe evacuation of the cavity has been also known (Japanese PatentPublication No. 1-46224). However, this technique cannot sufficientlysuppress the casting defects caused by the residual gases, because theinner pressure of the die cavity is maintained merely at a vacuumranging from about 200 to 400 millibar.

[0008] In view of the above circumstance, the inventors has developed adie-casting method comprising the steps of evacuating a die cavity to avacuum of 100 millibar or less, then injecting a reactive gas such asoxygen gas into the cavity, and initiating a forcible injection ofmolten aluminum alloy at a time the internal pressure of the cavity isincreased to atmospheric pressure or more (Japanese Patent ApplicationNo. 11-154566). Evacuating the cavity to the vacuum of 100 millibar orless accelerates vaporization of water from a release agent attached onthe inner surfaces of dies. Subsequently supplying the reactive gas tothe evacuated cavity allows the reactive gas to be spread all over theinterior of the dies, so as to effectively purge the residual air andthe water from the release agent as well as other undesirable gases inthe cavity. Thus, a die-cast product can be obtained with asignificantly reduced volume of incorporated gases, and thereby thecasting defects caused by the residual gases can be suppressed. Inaddition, the die-cast product can be heated without generation ofblisters caused by the residual gas. This allows the die-cast product tobe improved in its mechanical properties through a heat treatment suchas a T6 treatment.

[0009] Most of the reactive gas supplied to the cavity reacts with themolten aluminum ally injected into the cavity to form a fine structureof Al₂O₃ dispersed over the resulting product, and a part of thereactive gas is pushed out of the cavity by the molten aluminum alloyforcibly injected into the cavity. However, depending on theconfiguration of an intended die-cast product, a part of the reactivegas can be left in the cavity after the completion of the injection ofthe molten aluminum alloy. Then, the residual reactive gas isundesirably incorporated into the die-cast product without effectivereaction with the molten aluminum alloy or in an unreacted state. Forinstance, in dies intended to manufacture a die-cast product having acomplicated configuration, its cavity is typically designed to have aconfiguration in which a metal flow channel are branched into pluralchannels and the plurality of channels are jointed together. Thisjunction area creates a dead-end-like portion where a path for pushingout the reactive gas therethrough is clogged by the respective metalflows in the jointed channels to trap the reactive gas.

[0010] A system operable to evacuate a die cavity by opening a bypasspassage in fluid communication with the cavity (Japanese PatentPublication No. 1-46224) has a limited vacuum ranging from 200 to 400millibar in the cavity for the following reasons, resulting in asubstantial volume of residual air and insufficient purge of thereactive gas in an unreacted state, which leads to insufficientreduction of gases incorporated in a die-cast product.

[0011] First, the system is operated to evacuate gases directly from thecavity. Thus, it is required to spend substantial time for evacuatinggases from sleeve and runner regions through narrow gates. Further, thesystem is operated to evacuate gases initially from the cavity. Thus,the resultingly reduced pressure facilitates vaporization of a sleevelubricant and then sucks the vaporized sleeve lubricant into the cavity,resulting in increased humidity in the cavity. This high humidity canprovide water capable of reacting with molten aluminum alloy injectedinto the cavity to form hydrogen to be incorporated into a die-castproduct. A part of the sleeve lubricant can also be sucked into thecavity in a liquid state to cause contamination in the cavity.

[0012] Secondly, the system includes an evacuation device adapted to beselectively brought into fluid communication with the cavity through avalve device. That is, if the evacuation operation is continued untilthe completion of the operation of injecting the molten aluminum alloy,the molten metal can run in the evacuation device through a gap of theopened valve. Thus, it is required to terminate the evacuation of thecavity by shutting the valve before the molten aluminum alloy reachesthe valve. As a result, the residual gases cannot be sucked until thecompletion of the operation of injecting the molten aluminum alloy, andthe reactive gas in an unreacted state tends to be left in the cavity.

[0013] Further, the evacuation operation and the reactive-gas injectionoperation are performed by use of a common opening. In other words, itis impossible to inject the reactive gas simultaneously with theevacuation of the cavity, and thereby the reactive gas will be injectedafter terminating the evacuation of the cavity. Thus, after thetermination of the evacuation, some ambient air can enter into thecavity through a gap between mating faces of the dies and tends toremain therein. In addition, since the opening for injecting thereactive gas is provided only in the cavity, the sleeve cannot be filledwith the reactive gas, which undesirably allows ambient air to enterinto the cavity through a gap between the sleeve and the tip of theplunger.

DISCLOSURE OF THE INVENTION

[0014] The present invention has been made to solve the aforementionedproblems. It is therefore an object of the present invention to providean improved vacuum-oxygen die-casting method capable of utilizing bothadvantages of the conventional vacuum and oxygen die-casting methods andobtaining a die-cast product usable as not only structural members butalso functional members, by re-evacuating a die cavity through anoverflow region and a runner or sleeve of a die-casting machine duringan operation of forcible injecting molten aluminum to discharge areactive gas in an unreacted state from the cavity so as to achieve asignificantly reduced volume of gases incorporated into the die-castproduct as compared with conventional die-casting products.

[0015] In order to achieve this object, according the present invention,there is provided a die-casting method comprising the steps ofevacuating a cavity defined by dies of a die-casting machine to providetherein a vacuum of 100 millibar or less during a first period,injecting a reactive gas from a sleeve of the die-casting machine intothe cavity during a second period which has a partial overlap periodwith the first period and follows the overlap period, so as to increasethe inner pressure of the cavity to atmospheric pressure or more,pouring a molten aluminum alloy into the sleeve while keeping theinjection of the reactive gas, and subsequently moving a plunger in thesleeve forward to forcibly inject the molten aluminum alloy from thesleeve into the cavity while re-evacuating the cavity.

[0016] Preferably, the evacuation is performed at a suction speed of 500millibar/second or more. The cavity may be evacuated through anevacuation passage opened to a runner region in dies of a die-castingmachine, optionally in combination with an evacuation passage opened toan overflow region in the dies. The reactive gas such as oxygen gas isinjected into the cavity during the time-period which has the partialsimultaneous period with the evacuation period and follows thesimultaneous period, so as to increase the inner pressure of the cavityto atmospheric pressure or more. In this reactive-gas injectionoperation, it is preferable to inject a dehumidified reactive gas tokeep the cavity in humidity of 15% RH or less. The reactive-gasinjection operation may be terminated before the plunger is movedforward or may be continued until the completion of an entiredie-casting operation.

[0017] Molten aluminum alloy to be cast is poured from a pour opening ofthe sleeve to the interior of the sleeve and then forcibly injected intothe cavity by the forward movement of the plunger. Preferably, duringthis molten-aluminum-alloy injection operation, the plunger istemporarily stopped just after the plunger tip passes over the pouropening of the sleeve.

[0018] The re-evacuation operation may be initiated after the completionof the operation of pouring the molten aluminum alloy, and continueduntil the completion of the entire die-casting operation. During there-evacuation operation, the cavity is evacuated through the gas passageopened to the overflow region, optionally in combination with the gaspassage opened to the runner region.

[0019] The present invention also provides a die-casting machinesuitable for the above method. The die-casting machine comprises: anevacuation device having a discharge passage opened to a runner regionwhich guides molten aluminum alloy poured into a sleeve to a cavitydefined by dies, and a gas passage opened to an overflow region in thedies; and a reactive-gas supply device having a gas supply passageopened to a gas inlet provided in the sleeve closer to the dies than apour opening for pouring the molten aluminum alloy into the sleevetherethrough.

[0020] Preferably, a chill vent is provided between the opening of thegas passage and the overflow region, to prevent the molten metal fromleaking to the evacuation device. An applicable technique for assuringair-tightness between the cavity and ambient air may include a packinginterposed between respective mating faces of stationary and movabledies of the die-casting machine with surrounding the cavity, and agroove which is formed in at least one of the mating faces of thestationary and movable dies with surrounding the cavity and is in fluidcommunication with the evacuation device. Further, in order to assureair-tightness for an ejector pin slidably inserted in a pin insertionhole which is penetratingly provided in the movable die and opened tothe cavity, it is effective to provide a sealing device between theinner surface of the pin insertion hole and the outer surface of theejector pin.

[0021] The gas passage opened to the overflow region may be branchedinto a discharge passage selectively brought into fluid communicationwith the evacuation device and an air supply passage selectively broughtinto fluid communication with a compressed-air supply device, in orderto use the gas passage commonly for evacuating the cavity and supplyinga compressed air. A pressure gage for measuring the internal pressure ofthe cavity and a hygrometer for measuring humidity in the cavity may beprovided at the gas passage. Preferably, a drier is interposed in thegas supply passage extending from the reactive-gas supply device to thegas inlet of the sleeve.

[0022] In the conventional die-casting methods, gases contained in adie-cast product are derived from a residual air in a die cavity. Thevolume of the residual air can be significantly reduced by the vacuummethod or the oxygen die-casting method. However, even if the residualair in the cavity is reduced, a die-cast product obtained from suchmethods is ineluctably involved with the casting defects caused by theresidual gases. In the vacuum-oxygen die-casting method proposed byJapanese Patent Application No. 11-154566, the internal pressure of thecavity is reduced down to the vacuum of 100 millibar or less during theevacuation operation to facilitate vaporization of water from a releaseagent or the like, and then the reactive gas is supplied to theevacuated cavity to distribute the reactive gas all over the cavity. Bysupplying the reactive gas to increase the inner pressure of the cavityto atmospheric pressure or more, ambient air is prevented from enteringinto the cavity, and the water vapor from the release agent isadvantageously effused to outside.

[0023] In order to depressurize the cavity to the vacuum of 100 millibaror less during the evacuation operation, it is required to provide anairtight structure for the mating faces of the dies, the pour openingand the overflow region. The airtight structure also acts to reliablyhold the injected reactive gas in the cavity during the subsequentreactive-gas injection operation so as to keep the cavity in an oxygenatmosphere having a pressure of atmospheric pressure or more. Theinjected reactive gas does not react altogether with the molten aluminumalloy to form Al₂O₃, but a part of the reactive gas remains in anunreacted state in the cavity. The unreacted reactive gas is pushed intothe overflow region by the molten aluminum alloy forcibly injected intothe cavity. However, depending on the configuration of the die cavity, apath for pushing out the unreacted reactive gas can be clogged by themetal flows. In this case, a part of the unreacted reactive gas will beincorporated into a die-cast product. This tendency is stronglydeveloped in a die cavity having a configuration in which metal flowchannels are complicatedly branched and jointed.

[0024] In the present invention, the gas passages opened to the overflowand runner regions are selectively brought into fluid communication withthe evacuation device to evacuate the reactive gas from the cavityduring the molten-aluminum-alloy injection operation. Thus, the volumeof the unreacted gas incorporated in the molten aluminum alloy issignificantly reduced. Further, the evacuation is performed through thegas passage opened to the overflow region. This allows the reactive gasto be injected simultaneously with the evacuation operation, and allowsthe unreacted reactive gas to be continuously discharged until thecompletion of the molten-aluminum-alloy injection operation. Thus, thevolume of the residual unreacted gas in the cavity will be significantlyreduced. During the above evacuation operation, the runner region may beadditionally evacuated through the gas passage opened thereto.

[0025] Since the unreacted reactive gas is sucked based on evacuation,paths in fluid communication with the evacuating device to evacuate theunreacted reactive gas from the cavity are quickly established beforesuch paths are clogged by the metal flows. Thus, the unreacted reactivegas is not confined in the paths. Further, the runner region can beevacuated just before the molten aluminum alloy enters in the runnerregion.

[0026] In this respect, the technique of directly evacuating anunreacted reactive gas from a die cavity through the bypass passageopened to the cavity, (Japanese Patent Publication 1-46224) cannotachieve the vacuum of 100 millibar or less during the evacuationoperation. Thus, it is difficult to prevent the entry of ambient air andthe remanence of the unreacted reactive gas, and consequently a die-castproduct having a complicated configuration is often involved with thecasting defects caused by the unreacted reactive gas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a schematic diagram of a die-casting machine employing avacuum device and a reactive-gas supply device;

[0028]FIG. 2 is a schematic sectional view of the die-casting machine ofFIG. 1, viewed from the axial direction of a plunger thereof;

[0029]FIG. 3 is an explanatory view of the operating positions of theplunger.

BEST MODE FOR CARRYING OUT THE INVENTION

[0030] With reference to the drawings, the present invention will now bedescribed in conjunction with a specific embodiment.

[0031] A die-casting method according to an embodiment of the presentinvention comprises the steps of evacuating a cavity defined by dies ofa die-casting machine to provide therein a vacuum of 100 millibar orless, then injecting oxygen gas into the cavity to increase the innerpressure of the cavity to atmospheric pressure or more, and forciblyinjecting molten aluminum alloy while re-evacuating the cavity(hereinafter referred to as “DVO process”).

[0032]FIG. 1 schematically shows an exemplary die-casting machine usedto implement the DVO process. A cavity 30 is defined between astationary die 10 and a movable die 20 to form a profile correspondingto the configuration of an intended product. A runner 11 is formed inthe stationary die 10. The runner 11 is in fluid communication with asleeve 40 to allow a molten aluminum alloy M poured from a pour opening41 into the sleeve 40 to be forcibly injected into the cavity 30 by aplunger 42. A plurality of gates 12 (FIG. 2) are formed in accordancewith the configuration of the intended product and provides fluidcommunication between the runner 11 and the cavity 30 to allow themolten aluminum alloy M to be distributed over adequate portions of thecavity 30.

[0033] The cavity 30 includes an overflow portion 31 formed in themating face of the stationary or movable die 10, 20. A chill vent 32 isprovided outside the overflow portion 31. The overflow portion 31 actsto stabilize the flow of the molten aluminum alloy M in the cavity 30.As illustrated, the chill vent 32 is formed by providing a rugged orwavy shaped portion in a part of the mating face of each of thestationary and movable dies 10 and 20, and acts to facilitatesolidification of the molten aluminum alloy M to be brought into contacttherewith so as to prevent the molten aluminum alloy M from being suckedtoward an after-mentioned evacuation device. By virtue of the chill vent32, the cavity 30 can be evacuated without run-in or leakage of themolten aluminum alloy M during the injecting the molten aluminum alloyM.

[0034] An ejector pin 21 is provided movably or slidably in the movabledie 20 to release a die-cast product therefrom.

[0035] A packing 51 such as an O-ring is interposed between the matingfaces of the stationary and movable dies 10 and 20 to keep the cavity 30airtight against ambient air. The packing 51 is fitted in a groovesurrounding the cavity 30 to sealingly prevent ambient air from enteringthrough a gap between the stationary and movable dies 10 and 20. Anotherpacking 52 is provided at a pin insertion hole 22 into which the ejectorpin 21 is slidably inserted. The packing 52 acts to maintainair-tightness between the inner surface of the pin insertion hole 22 andthe outer surface of the ejector pin 21. These packings 51 and 52provide an effective sealing effect allowing the cavity 30 to bedepressurized to the vacuum of 100 millibar or less and to be evacuatedduring the operation of injecting the molten aluminum alloy M.

[0036] In order to provide an enhanced effect on preventing ambient airfrom entering into the cavity, the groove having the packing 51 receivedtherein may be brought into fluid communication with and evacuated by anevacuation device 60. In addition to the groove having the packing 51received therein, an evacuation groove 69 (FIG. 2) without any packingmay be provided in at least one of the mating faces between thestationary and movable dies 10 and 20 with surrounding the cavity 30,and may be brought into fluid communication with and evacuated by to theevacuation device 60.

[0037] A discharge passage 61 in fluid communication with the evacuationdevice 60 is opened to the runner 11 to evacuate the cavity 30. A vacuumvalve 63 is provided at the opening of the discharge passage 61 facingto the runner 11. The vacuum valve 63 is actuated by a driving cylinder62 to open and close the opening. Another discharge passage 65 isbranched from a gas passage 64 opened at a position between the chillvent 32 and the packing 51 on the mating surfaces of the stationary andmovable dies 10 and 20. The discharge passage 65 is brought into fluidcommunication with the evacuation device 60 through a vacuum valve 66.

[0038] A pressure gage 67 is provided at the gas passage 64 to detectthe internal pressure of the cavity 30. Preferably, a hygrometer 68 isprovided at the gas passage 64 to control humidity in the cavity 30.

[0039] An air supply passage 71 is branched from the gas passage 64 andbrought into fluid communication with a compressed-air supply device 70through a check valve 72. Thus, the gas passage 64 is also used tosupply a compressed air into the cavity 30. When the dies are openedafter the completion of an entire die-casting operation, a compressedair is supplied through the gas passage 64 to remove foreign substancesfrom the passages and the like associated with the evacuation.

[0040] A reactive-gas supply device 80 is provided to inject a reactivegas such oxygen gas into the cavity after evacuating the cavity 30 inthe DVO process. The reactive gas is supplied into the sleeve 40 fromthe reactive-gas supply device 80 through a gas supply passage 81 and agas inlet 82. A drier 83 is interposed in the gas supply passage 81 todehumidify the reactive gas and keep the cavity 30 in lower humidity.

[0041] The internal pressure and humidity in the cavity are detected,respectively, by the pressure gage 67 and the hygrometer 68 which areprovided at the gas passage 64. The detected value from the pressuregage 67 is sent to a control unit for controlling respective operationsof the evacuation device 60, the compressed-air supply device 70 and thereactive-gas supply device 80, to control the evacuation, oxygen-gasinjection and re-evacuation periods. The control unit is also adapted toinitiate the supply of molten aluminum alloy M to the sleeve 40, at thetime the detected value from the hygrometer 68 is 15% RH or less and theinternal pressure in the cavity 30 is atmospheric pressure or more.

[0042] The DVO process according to the present invention will bedescribed below.

[0043] The movable and movable dies 10 and 20 are clamped, and then thecavity 30 is evacuated through the runner 11. The cavity 30 is alsoevacuated by using the gas passage 64 opened to the mating faces betweenthe chill vent 32 and the packing 51. The evacuation operation iscontinued until the pressure gage 67 detects that the internal pressureof the cavity 30 is reduced down to a vacuum of 100 millibar or less.During the evacuation operation, the plunger tip 43 is located at agiven position, or an evacuation initiation position, between the pouropening 41 and the gas inlet 82 of the sleeve 40 to prevent ambient airfrom sucking into the sleeve through the pour opening 41 (FIG. 3). Sincethe evacuation is performed through the runner 11, a lubricant in thesleeve 40 is discharged to outside without entering in the cavity 30.

[0044] During the evacuation operation, the suction speed is preferablyarranged at 500 millibar/second or more. Even if the cavity 30 has acomplicated configuration, the suction speed of 500 millibar/second ormore allows undesirable gases to be removed from all over the cavity 30.Further, when the cavity 30 is evacuated at the suction speed of 500millibar/second or more, water contained in a release agent attached onthe inner surface of the dies 10, 20 is effectively purged by virtue ofa bumping phenomenon, and thereby the water in the cavity 30 issignificantly reduced.

[0045] Preferably, the evacuation operation is continued for about 1 to2 seconds while isolating the pour opening port 41 from the cavity bythe plunger 42. In this respect, the time-period of the evacuation isarranged relatively longer as compared with the conventional vacuumdie-casting method in which the evacuation period is less than 1 secondwithout isolating the pour opening from the cavity. The inner pressureof the cavity 30 is reduced down to the vacuum of 100 millibar or lessduring the evacuation operation. The water contained in the releaseagent on the inner surface of the dies is separatedly vaporized anddischarged from the cavity 30 to outside by the evacuation.

[0046] The conventional evacuation having the vacuum higher than 100millibar is involved with a relatively large volume of air remaining inthe cavity 30. The residual air is insufficiently replaced by a reactivegas during the subsequent reactive-gas injection operation and isincorporated into a die-cast product, resulting in occurrence of castingdefects such as blowholes or blisters. On the other hand, the ultimatevacuum of 100 millibar or less can effectively facilitate vaporizing thewater contained in the release agent or the like and discharging theresulting water vapor to outside. In particular, when the evacuation isperformed at a suction speed of 500 millibar/second or more, thevaporization of the water in the release agent or the like isaccelerated by the bumping phenomenon, and thereby the residual water issignificantly reduced. In view of the ability of the evacuation device,the maximum suction speed would be about 800 millibar/second.

[0047] After the cavity 301 is evacuated to the vacuum of 100 millibaror less, a reactive gas is injected from the gas inlet 82 to the cavity30. The reactive-gas injection period has a partial overlap period withthe evacuation period, and the evacuation operation is terminated afterthe completion of the overlap period. During this partial overlapperiod, the injected reactive gas is distributed all over the cavity 30,and ambient air is prevented from entering into the cavity through thegap between the mating faces of the dies. The reactive gas injectionoperation is continued until the pressure gage 67 detects that theinternal pressure of the cavity 30 is increased to the atmosphericpressure or more.

[0048] During the reactive-gas injection operation, humidity in thecavity 30 is measured by the hygrometer 68 and is controlled so as notto go over 15% RH. This humidity control can reduce the amount of waterwhich enters into the cavity 30 together with the reactive gas andgenerates hydrogen gas by reacting with the molten aluminum alloy M.Preferably, the reactive gas is passed through the driers 3, and thedried reactive gas is injected into the cavity to reduce humidity in thecavity 30

[0049] After the internal pressure of the cavity 30 is increased toatmospheric pressure or more, the plunger tip 43 is moved backward to amolten-aluminum-alloy pouring position (FIG. 3) for providing fluidcommunication between the pour opening 41 and the cavity. Then, themolten aluminum alloy M is poured into the sleeve 40 in a mountnecessary for one shot or one die-casting operation. Since the internalpressure of the cavity 30 is kept at the atmospheric pressure or moreduring this poring operation, the reactive gas is blown out through thepour opening 41 to prevent ambient air from entering in the sleeve.After the completion of the operation of pouring the molten aluminumalloy M, the fluid communication between the pour opening 41 and thecavity 30 is blocked by moving the plunger 42 forward.

[0050] After the pouring operation, the cavity 30 is re-evacuatedthrough the overflow portion 31. This re-evacuation may be performed incombination with the evacuation through the discharge passage 61 openedto the runner 11. Preferably, the re-evacuation period is arranged tohave a partial overlap period with the reactive-gas injection period.The reactive-gas injection operation may be continued until thecompletion of the entire die-casting operation. During this overlapperiod, an excessive reactive gas is discharged from the cavity 30 tooutside, and water from the release agent and/or lubricant is dischargedfrom the cavity 30 together with the reactive gas. This also eliminatesthe disadvantage, or ambient-air entry, likely caused by re-evacuatingafter the completion of the reactive-gas injection operation (JapanesePatent Publication No. 1-46224).

[0051] The plunger 42 is moved forward at a low speed until the plungertip 43 passes over the pour opening 41 and reaches a given position, ora high-speed injection initiation position, for initiating a high-speedinjection (FIG. 3), while re-evacuating the cavity 30. In the course ofthe forward movement of the plunger 42, the re-evacuation operation isinitiated at a time the plunger tip 43 reaches at the aforementionedevacuation initiation position. The forward movement of the plunger 42may be temporarily stopped at the evacuation initiation position. Thetemporary stop of the plunger 42 provides a longer evacuation periodcorresponding to the stop period to allow undesirable gases and watervapor to be further discharged from the cavity 30.

[0052] Then, the plunger 42 is moved forward at a high speed from thehigh-speed injection initiation position to an injection limit positionto forcibly inject the molten aluminum alloy M into the cavity 30. Sincethe cavity 30 is continuously evacuated during thismolten-aluminum-alloy injection operation, the reactive gas in anunreacted state is effectively removed from the cavity 30 as the moltenaluminum alloy M is forcibly injected into the cavity. Even if thecavity 30 has a configuration in which a plurality of metal flowchannels are complicatedly branched and joined, desirable flow paths ofthe reactive gas are quickly established to provide fluid communicationwith the evacuation device. Thus, the flow paths in fluid communicationwith the evacuation device are not clogged by the molten aluminum alloy,which prevents the unreacted reactive gas to be trapped by andincorporated into the molten aluminum alloy. The re-evacuation operationis continued until the cavity 30 is filled with the molten aluminumalloy M.

[0053] After the completion of a die-casting operation, there-evacuation operation is terminated, and then the dies are opened.Then, a compressed air is supplied from the compressed-air supply device70 through the air supply passage 71 and the gas passage 64 to removeforeign substances from the passages and the like associated with theevacuation.

[0054] By arranging the timings between the evacuation, reactive-gasinjection and re-evacuation operations as described above, residualgases and unreacted reactive gas are sufficiently removed from thecavity 30. Further, the reactive gas in the cavity 30 adequately reactswith the molten aluminum alloy M to provide a die-cast product having aless volume of incorporated gases. The obtained die-cast product has asignificantly reduced volume of gases, and thereby can be subjected to aheat treatment such as a T6 treatment to provide enhanced mechanicalproperties without blisters caused by expansion of these gases.

EXAMPLE 1

[0055] An example will be described in which the present invention isapplied to a die-casting machine with dies adapted to define athin-box-shaped cavity 30 having a plurality of ribs partitioning theinterior thereof and a cooling device capable of partially water-coolingthe dies.

[0056] Dies were prepared in which an ejector pin 21 and a pin-insertionhole 22 are air-tightly sealed therebetween and respective mating facesof stationary and movable dies 10 and 20 are air-tightly sealed with apacking 51, and the dies were then installed in a die-casting machine.The dies were heated to 180 degrees centigrade, and a release agent wasapplied on the inner surfaces of the dies for 5 seconds.

[0057] Then, a pour opening 41 of a sleeve 40 was isolated from thecavity by a plunger 42, and gases were evacuated from the cavity 30 andthe sleeve 40 through a discharge passage 61 and a gas passage 64 at asuction speed of 700 millibar/second for 1.5 seconds. The evacuationoperation was continued until a pressure gage 67 indicates a pressure of75 millibar.

[0058] Oxygen gas or a reactive gas is injected from a reactive-gassupply device 80 into the sleeve 40, while keeping the evacuation. Theevacuation operation was terminated after the lapse of 2 seconds fromthe initiation of the oxygen gas injection, while the oxygen gas wascontinuously injected until the internal pressure of the cavity 30 isincreased to the atmospheric pressure or more.

[0059] After atmospheric pressure or more of the internal pressure and15% RH or less of the humidity in the cavity 30 were confirmed,respectively, by the pressure gage 67 and a hygrometer 68, the plunger42 was moved backward to provide fluid communication between the pouropening 41 and the cavity, and then molten aluminum alloy M was pouredinto the sleeve 40. The oxygen-gas injection operation was continuedduring the molten-aluminum-ally pouring operation. Subsequently, theoxygen-gas injection operation was terminated just after the sleeve 40was completely filled with the molten aluminum alloy M or wasuninterruptedly continued until the completion of an entire die-castingoperation.

[0060] After the molten aluminum alloy M was poured into the sleeve 40,the plunger 42 was moved forward to a high-speed-injection initiationposition (FIG. 3) and then stopped thereat for 1 second. The plunger 42was then moved forward at a high speed to forcibly inject the moltenaluminum alloy M into the cavity 30 at an injection speed of 3 m/second.During the operation of injecting the molten aluminum alloy M, thecavity 30 was re-evacuated through the discharge passage 64 and the gaspassage 64, and the re-evacuation was terminated after the completion ofthe entire die-casting operation.

[0061] The dies were opened after the lapse of 20 seconds from thecompletion of the entire die-casting operation, and then a compressedair was supplied from the compressed-air supply device 70 to clean upthe inside of the gas passage 64. Then, a resulting die-cast product wastaken out of the dies by sticking out the ejector pin 21.

[0062] Table 1 shows respective timings of the evacuation, oxygen gasinjection, and re-evacuation operations in the aforementioned DVOprocess together with the operations of pouring and forcibly injectingthe molten aluminum alloy M, based on the lapsed time from theinitiation of the evacuation operation. The process in Example 3includes a step of temporarily stopping the plunger 42, and thus has alonger period between the completion of pouring the molten metal in thesleeve 40 and the initiation of forcibly injecting the molten metal intothe cavity 30, corresponding to the stop period, as compared withExample 1 and 2. Table 1 also shows Comparative Example 4 in which theevacuation, oxygen gas injection and re-evacuation operations weresequentially switched without any overlap period therebetween. TABLE 1Lapsed Time in each Die-Casting Operation (sec) Comparative InventiveExample Example operation Example 1 Example 2 Example 3 Example 4evacuation 10 10 10 10 termination injecting oxygen gas initiation 8 7 811 termination 24 23 24 26 pouring molten metal to sleeve initiation 2020 20 27 termination 22 22 22 28 injecting molten metal to cavityinitiation 24 23 26 29 termination 26 25 28 31 re-evacuation initiation24 23 24 29 termination 27 26 28 31 opening dies 70 70 75 75

[0063] The volume of residual gases contained in each of obtaineddie-cast products was measured by the Ransley method, and theirmechanical properties were measured.

[0064] As seen in the measurement result of Table 2, as compared withthat from Comparative Example 4, each of the die-cast products fromExamples 1 to 3 has a remarkably reduced volume of gases such as N₂ andH₂, and exhibits higher values in elongation and tensile strength.Further, by virtue of the extremely reduced volume of gases, each of thedie-cast products from Examples 1 to 3 could be improved in mechanicalproperties by subjecting to a heat treatment such as a T6 treatment(heating at 510 degrees centigrade for 3 hours—water quenching—aging at1600 degrees centigrade for 5 hours) without occurrence of blisters. Onthe other hand, after the T6 treatment, the die-cast product fromComparative Example 4 exposed blisters at junctions of metal flowsarising during the die-casting operation, and proved that some gaseswere incorporated therein. TABLE 2 Influence of Difference inManufacturing Conditions on Properties of Die-Cast Products and Die-CastProducts Subjected to T6 Treatment die-cast product die-cast productsubjected (w/o heat treatment) to T6 treatment Example gas volumetensile strength elongation tensile strength elongation No. (cc/11g-Al)(kg/mm²) (%) (kg/mm²) (%) note 1 0.8 25 9 30 14 Inventive 2 0.9 25 11 3015 Example 3 0.6 27 12 32 16 4 1.8 24 7 28 10 Comparative Example

INDUSTRIAL APPLICABILITY

[0065] As described above, in the present invention, a die-cast productis produced by supplying molten aluminum alloy to a cavity whilesequentially performing the evacuation, reactive-gas injection andre-evacuation operations with respective overlap periods therebetween.In the evacuation operation, the internal pressure of the cavity isreduced down to the vacuum of 100 millibar or lower, to discharge airfrom the cavity. Then, in the reactive-gas injection operation, theinternal pressure of the cavity is increased to atmospheric pressure ormore, to completely remove the residual air in the cavity and othergases such as water vapor attached on the inner surface of the dies.Further, the re-evacuation operation is performed in synchronous withthe operation of forcibly injecting the molten aluminum alloy into thecavity to remove the unreacted reactive gas from the cavity and preventthe unreacted reactive gas from being incorporated into the moltenaluminum alloy, so as to provide a die-cast product having asignificantly reduced volume of gases. The die-cast product obtained inthis manner is free from the casting defects such as internal porositiesor blowholes caused by the incorporated gases, and can be subjected to aheat-treatment for improving its mechanical properties. Thus, thedie-casting method according to the present invention can provide animproved die-cast product usable even for functional members with itsintrinsic advantage of excellent productivity.

What is claimed is:
 1. A die-casting method comprising the steps of:evacuating a cavity defined by dies of a die-casting machine to providetherein a vacuum of 100 millibar or less during a first period;injecting a reactive gas from a sleeve of said die-casting machine intosaid cavity during a second period which has a partial overlap periodwith said first period and follows said overlap period, so as toincrease the inner pressure of said cavity to atmospheric pressure ormore; pouring a molten aluminum alloy into said sleeve while keeping theinjection of said reactive gas, and subsequently moving a plunger insaid sleeve forward to forcibly inject said molten aluminum alloy fromsaid sleeve into said cavity, while re-evacuating said cavity through agas passage opened to an overflow region in said cavity.
 2. Adie-casting method as defined in claim 1, wherein said cavity isevacuated at a suction speed of 500 millibar/second or more.
 3. Adie-casting method as defined in claim 1, wherein said cavity isevacuated through a runner region in said dies.
 4. A die-casting methodas defined in claim 1, wherein said cavity is evacuated through a runnerregion in said dies in addition to said overflow region.
 5. Adie-casting method as defined in claim 1, wherein said reactive gas isinjected after being dehumidified, to keep said cavity in humidity of15% RH or less.
 6. A die-casting method as defined in claim 1, whereinsaid plunger is moved forward after the termination of said reactive gasinjecting step.
 7. A die-casting method as defined in claim 1, whereinsaid reactive gas injection step is continued until the completion of anentire die-casting operation.
 8. A die-casting method as defined inclaim 1, wherein said plunger is temporarily stopped just after a tip ofsaid plunger tip passes over an pour opening of said sleeve.
 9. Adie-casting method as defined in claim 1, wherein said cavity iscontinuously re-evacuated until the completion of an entire die-castingoperation.
 10. A die-casting method as defined in claim 1, wherein saidcavity is re-evacuated through said overflow region.
 11. A die-castingmachine comprising: a sleeve for receiving therein molten aluminum alloypoured through a pour opening thereof; a plunger adapted to becontrollably moved forward and backward in said sleeve; dies including aset of stationary and movable dies adapted to be brought into airtightcontact with one another to define therebetween a cavity into which saidmolten aluminum alloy is forcibly injected through said sleeve and arunner in said dies, said dies being optionally formed with a chill ventand/or a groove in the mating faces thereof; at least one of ejector pinslidably inserted into a pin insertion hole and adapted to be projectedinto said cavity, said pin insertion hole being penetratingly providedin said movable die; an evacuation device having a discharge passageopened to the runner region and a gas passage opened to the overflowregion; and a reactive-gas supply device having a gas supply passageopened to a gas inlet provided in said sleeve closer to said dies thansaid pour opening of said sleeve.
 12. A die-casting machine as definedin claim 11, which further includes a packing interposed between saidmating faces of said stationary and movable dies with surrounding saidcavity.
 13. A die-casting machine as defined in claim 11, which furtherincludes a groove is formed in at least one of said mating faces of saidstationary and movable dies with surrounding said cavity, said groovebeing in fluid communication with said evacuation device.
 14. Adie-casting machine as defined in claim 11, wherein said ejector pin isslidably inserted into said pin insertion hole penetratingly provided insaid movable die and opened to said cavity, wherein the inner surface ofsaid pin insertion hole and the outer surface of the ejector pin areair-tightly sealed therebetween.
 15. A die-casting machine as defined inclaim 11, wherein said gas passage opened to said overflow region isbranched into a discharge passage selectively brought into fluidcommunication with said evacuation device and an air supply passageselectively brought into fluid communication with a compressed-airsupply device for supplying a compressed air.
 16. A die-casting machineas defined in claim 11, wherein said gas passage is provided with apressure gage for measuring the internal pressure of said cavity.
 17. Adie-casting machine as defined in claim 11, wherein said gas passage isprovided with a hygrometer for measuring humidity in said cavity
 18. Adie-casting machine as defined in claim 11, which further includes adrier interposed in said gas supply passage extending from saidreactive-gas supply device to said gas inlet of the sleeve.